Properties and microstructure of ultrathin (La,Ca)MnO3 films under different conditions of strain

Abstract

Ultrathin films of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(x\ensuremath{\cong}0.33)$ in a thickness range below $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were deposited by direct current (dc) magnetron sputtering on different substrates, $(001)\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (STO), $(001)\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (LAO), and $(110)\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_{3}$ (NGO) in order to study the effects of strain in this thickness regime. The combined study of physical properties and microstructure by high-resolution electron microscopy (HREM) reveals that ultrathin $(\ensuremath{\lesssim}6\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ strained films on STO can be grown in a body-centered-tetragonal structure not recognized previously. Such films do not show an insulator-metal (IM) transition. At slightly larger thickness, around $6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the microstructure is different. Depending on the growth parameters we find either a structure involving $\mathrm{Mn}{\mathrm{O}}_{6}$-octahedra tilted around two axes, in combination with the occurrence of twin boundaries as reported previously; or the bulk orthorhombic $Pnma$ structure. In the latter case an IM transition is found at temperatures close to the bulk value. On lattice-matched NGO, the IM transition and bulk-like magnetoresistance effects are observed down to the lowest thickness investigated $(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. On LAO, which compresses the lattice, no IM transition is found even at a thickness of $15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which is due to island-like growth, as confirmed by HREM observations.